Abstract

We have investigated the decay of optically-induced, short-period (≅120 nm) gratings in GaAs/AlGaAs multiple-quantum-well structures with a barrier thickness L_B of 1.4-15 nm. The crystals were excited with 100 fs pulses at wavelengths either resonant with the n = 1 exciton or into the bands. The decay of the diffraction efficiency of the gratings shows two components: a fast initial contribution that is independent of l_b and has a decay time of 200-300 fs and a slower exponential contribution that depends on l_b and has decay times between 1.5-90 ps. The latter component is determined by perpendicular transport by means of over-barrier hopping and phonon-assisted tunneling among adjacent wells and is consistent with a simple-rate-equation model based on these transport mechanisms and the recombination of the excited carriers. The fast initial component is observed only for resonant excitation and may be connected with the exciton ionization and/or reorientation. To investigate this feature, we performed two-beam self-diffraction experiments with cross-polarized pulses. Here, we observed signals in the background-free direction consistent with self-diffraction from an orientational grating, whereas we saw a fast increase of the transmission in the direction of the transmitted beam

© 1990 Optical Society of America